In the conventional wheel support bearing assemblies of the second and third generation types, in which the inner member is utilized as a rotatable member and the outer member as a stationary member, the outer member is generally formed with a vehicle body fitting flange on an outer periphery thereof and, therefore, the use is made of a medium carbon steel such as, for example, S53C as a material for the outer member to thereby secure a required strength. While the raceway surface in the outer member is generally subjected to the induction hardening to thereby secure a required strength in a rolling unit, a portion of the outer member other than the raceway surface is not specifically subjected to any heat treatment and is utilized as forged. The reason therefor is that although the tensile strength will increase if the outer member in its entirety is hardened, it will become fragile against deformation (elongation).
Also, in the outer member employed in the conventional wheel support bearing assemblies of the kind discussed above, emphasis has been placed on the proof strength against the plastic deformation, which takes place under a high load and at a low cycle, rather than the fatigue strength under a low load with a high cycle, since as compared with the inner member, which is the rotatable member, the frequency of repeated load is low.
However, in order to reduce the weight of the automotive vehicle, demands have been made to reduce the weight of the wheel support bearing assembly as well and the functionality comparable to that in the prior art wheel support bearing assembly is required even in the wheel support bearing assembly, which has been assembled compact in size. This equally applies even to the outer member employed in the wheel support bearing assembly and it is quite often that the vehicle body fitting flange is reduced in size and weight (capacity).
To meet those demands, it has been suggested to apply the induction hardening to a root area of the pilot portion in the rotatable member (the hub axle, or the outer member in an outer ring rotating model of the wheel support bearing assembly of the second generation), or to thermally refine the rotatable member in its entirety, so as to increase the fatigue strength of a high stress site during the cornering of the automotive vehicle. (See, for example, the Patent Documents 1, 2 and 3 listed below.)    [Patent Document 1] JP Laid-open Patent Publication No. 2005-308152    [Patent Document 2] JP Laid-open Patent Publication No. 2006-291250    [Patent Document 3] JP Laid-open Patent Publication No. 2004-314820
It is quite frequently observed that the automotive vehicle skids by some reason during the travel thereof with its vehicle wheel (tire, wheel) colliding against the curbside. At this time, an axially acting load may be unexpectedly imposed on the wheel support bearing assembly and a moment load acts thereon. The skidding of the automotive vehicle occurs when the road surface is wetted with rain or is ice covered. When the road surface is ice covered, the atmospheric temperature is low, and when the automotive vehicle then parked in the open air skids to collide against the curbside soon after the start of travel from the parked condition, the load is unexpectedly imposed on the wheel support bearing assembly while the latter is in a cold condition.
Exemplary cases of fracture that occur when the unexpected load acts on the wheel support bearing assembly includes brittle fracture and ductile fracture. In the case of the brittle fracture, the energy that can be absorbed is low and the toughness is low. On the other hand, in the case of the ductile fracture, the energy that can be absorbed is high and the toughness is high. In general when the Charpy impact test is conducted on a steel material, the fracture transition temperature, at which the brittle fracture surface and the ductile fracture surface assume 50% each appears. At temperatures below such transition temperature, the energy that can be absorbed is lower than at temperature above the transition temperature (susceptive to brittle fracture). Where the fracture transition temperature is high, it turns out that the material is low in toughness even at high temperature. On the other hand, if the fracture transition temperature is low, it turns out that the material is high in toughness even at low temperature.
If, as is the case with the above described wheel support bearing assembly, the carbon steel is used as a material for the outer member and such outer member is used in the form as forged and, particularly, with no heat treatment applied to that portion of the outer member other than the induction heated raceway surfaces, the shock absorption energy at a low temperature (−40 to −20° C.) decreases considerably at a rate higher than that at normal temperatures as shown in the chart of FIG. 11 showing results of the Charpy impact test. Some specific examples of measurements of the absorption energy are shown below and more specific measurements are shown in Table 1.
 20° C.35 to 40 J/cm2 [N = 6 average: 37.0 J/cm2]−20° C. 9 to 30 J/cm2 [N = 6 average: 20.8 J/cm2]−40° C. 8 to 21 J/cm2 [N = 6 average: 15.8 J/cm2]
TABLE 1Test Temperature (° C.)−40−2002040Absorptionn = 6 av.15.820.827.837.028.3EnergyMaximum21.030.034.040.040.0(J/cm2)Minimum8.09.019.035.036.0
It is to be noted that conditions employed during the Charpy impact test are as follows.
2 mm/U notch
For the purpose of imitating the structure after the hot forging, a hot-rolled steel bar made of S53C was held at 1050° C. for 1 hour and was then allowed to stand until it cooled.
A test piece was prepared from a center portion having a quater diameter of the bar of 60 mm in diameter (the longitudinal direction of the test piece conforms to the direction of rolling).
Although the material was carbon steel S53C, two types by different steel makers were used (the test piece was three for each type).
From the above described results of the Charpy impact test, it has been certain that when the impact load acts on the above described wheel support bearing assembly while the latter is in the cold condition, the outer member of the structure as forged is susceptible to the chilled load while the toughness is lowered at the low temperature and, therefore, there is the possibility that the vehicle body fitting flange, which has been made compact and lightweight, will be fractured to such an extent that the wheel support bearing assembly itself may depart from the automotive vehicle.